I advise against trying to bring port size out to 340/360 size. Here is a 360 gasket on my 273 Offy dual quad. If you bring it out to the 360 size you will have near paper thin metal afterward. My suggestion is to run it as is with 2 Eddie/AFB 500s, or go with a single carb set up. Other manufacturers may have left enough meat to grind away but it looks like Offy intended this manifold for the 273.